For many years Organic Light-Emitting Diodes (OLEDs) have been touted as the technology that will usurp conventional fluorescent tubes as the market's dominant lighting source, owing to their use of environmentally benign, relatively cheap emissive materials and their capacity to achieve external quantum efficiencies of 100%.
However, in spite of these desirable features, OLEDs have struggled to attain universal marketability as the solid-state lighting (SSL) technology of choice. The emissive materials employed are incapable of effecting balanced charge injection and mobility, thus necessitating the encapsulation of low work function, air-reactive electrodes within complex multilayer compositions. Fabrication of such sensitive devices thus typically requires vacuum sublimation—a process which is both labor- and cost-intensive, and requires thermally stable, non-ionic materials, which limits the choice of organometallic triplet harvesters that might be used.
A promising alternative lighting technology to OLEDs is Light-Emitting Electrochemical Cells (LEECs). By using charged materials they confer many of the same advantages but they allow for the circumvention of the arduous vacuum sublimation process. Processing is instead carried out by solution printing, using air-stable high work function electrodes in a single- or two-layer device architecture, making large-area artificial illumination a very real possibility.
Two classes of emitter materials are typically employed:                1) a mixture of conjugated polymer, ion transport material and inorganic salt such as LiOTf; or        2) an ionic Transition Metal Complex (iTMC). Of the different families of iTMCs, by far the most widely studied and exciting class of emitters for LEECs are heteroleptic cationic iridium(III) complexes, of the form [Ir(C^N)2(N^N)]+, where C^N is a monoanionic cyclometalating bis(chelate) and N^N is a neutral diimine ancillary ligand. More generally, the term C^N means a ligand coordinating by carbon and by nitrogen to the metal (iridium in this example). The term N^N means a ligand datively coordinating by two nitrogen atoms to the metal. A typical C^N ligand is 2-phenylpyridinato.        
LEECs too present their own design challenges. Issues that still require addressing for iTMC LEECs include slow turn-on times, limited device stability and poor colour quality. In particular, few examples exist of blue-emitting LEECs, which is mainly due to a shortage of deep blue, brightly emitting complexes. Blue emitters are critical both for white light emission and as a component of RGB-based pixels in displays.
Designing iridium complexes for blue emission by combining electron-deficient C^N and electron-rich N^N ancillary ligands, has been met with varying degrees of success. There are now a few reported examples of deep blue emitting cationic iridium complexes in solution (λmax<470 nm), but significant issues still remain regarding the brightness of these emitters.
Complexes bearing imidazole ligands have been employed in a diverse set of photo physical applications ranging from bio imaging and sensing to excited state proton-coupled electron transfer (PCET) and solid-state lighting.
Of particular interest are iridium complexes of the form [Ir(C^N)2(N^N)]+ bearing a 1H,1H′-2,2′-biimidazole (biim) N^N ligand.
Kim and co-workers recently showed that combining the biim N^N ligand with an electron-deficient C^N ligand in [Ir(dFpmpy)2(biim)]+, where dFpmpy is 2-(2′,4′-difluorophenyl)-4-methylpyridinato, could achieve deep blue emission, with emission maxima at 456 and 484 nm in DCM. However, despite such promising examples in terms of emission energy, photoluminescence quantum yields (PLQY, ΦPL) remain very low (Figure A below, reference 1).

Accordingly there is a need to provide improved and alternative iridium complexes for use in display and lighting uses, such as in light emitting electrochemical cells (LEECs).